In this paper, we examine the role of polarimetry in synthetic aperture radar (SAR) interferometry. We first propose a general formulation for vector wave interferometry that includes conventional scalar interferometry as a special case. Then, we show how polarimetric basis transformations can be introduced into SAR interferometry and applied to form interferograms between all possible linear combinations of polarization states. This allows us to reveal the strong polarization dependency of the interferometric coherence. We then solve the coherence optimization problem involving maximization of interferometric coherence and formulate a new coherent decomposition for polarimetric SAR interferometry that allows the separation of the effective phase centers of different scattering mechanisms. A simplified stochastic scattering model for an elevated forest canopy is introduced to demonstrate the effectiveness of the proposed algorithms. In this way, we demonstrate the importance of wave polarization for the physical interpretation of SAR interferograms. We investigate the potential of polarimetric SAR interferometry using results from the evaluation of fully polarimetric interferometric shuttle imaging radar (SIR)-C/X-SAR data collected during October 8-9, 1994, over the SE Baikal Lake Selenga delta region of Buriatia, Southeast Siberia, Russia.

Polarimetric SAR interferometry

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Stochastic Processes And Filtering Theory

This letter introduces a two-dimensional bilinear mapping operator referred to as the cubic phase (CP) function. For first-, second-, or third-order polynomial phase signals, the energy of the CP function is concentrated along the frequency rate law of the signal. The function, thus, has an interpretation as a time-frequency rate representation. The peaks of the CP function yield unbiased estimates of the instantaneous (angular) frequency rate (IFR) and, hence, can be used as the basis for an IFR estimation algorithm. The letter defines an IFR estimation algorithm and theoretically analyzes it. The estimation is seen to be asymptotically optimal at the center of the data record for high signal-to-noise ratios. Simulations are provided to verify the theoretical claims.

A new technique for instantaneous frequency rate estimation

In this paper, we consider the problem of joint delay-Doppler estimation of moving targets in a passive radar that makes use of orthogonal frequency-division multiplexing communication signals. A compressed sensing algorithm is proposed to achieve supper resolution and better accuracy, using both the atomic norm and the $\ell _1$-norm. The atomic norm is used to manifest the signal sparsity in the continuous domain. Unlike previous works that assume the demodulation to be error free, we explicitly introduce the demodulation error signal whose sparsity is imposed by the $\ell _1$-norm. On this basis, the delays and Doppler frequencies are estimated by solving a semidefinite program (SDP) which is convex. We also develop an iterative method for solving this SDP via the alternating direction method of multipliers where each iteration involves closed-form computation. Simulation results are presented to illustrate the high performance of the proposed algorithm.

Super-Resolution Delay-Doppler Estimation for OFDM Passive Radar

This article studies the processing principles, implementation challenges, and performance of orthogonal frequency-division multiplexing (OFDM)-based radars, with particular focus on the fourth-generation Long-Term Evolution (LTE) and fifth-generation (5G) New Radio (NR) mobile networks’ base stations and their utilization for radar/sensing purposes. First, we address the problem stemming from the unused subcarriers within the LTE and NR transmit signal passbands and their impact on frequency-domain radar processing. In particular, we formulate and adopt a computationally efficient interpolation approach to mitigate the effects of such empty subcarriers in the radar processing. We evaluate the target detection and the corresponding range and velocity estimation performance through computer simulations and show that high-quality target detection as well as high-precision range and velocity estimation can be achieved. In particular, 5G NR waveforms, through their impressive channel bandwidths and configurable subcarrier spacing, are shown to provide very good radar/sensing performance. Then, a fundamental implementation challenge of transmitter–receiver (TX–RX) isolation in OFDM radars is addressed, with specific emphasis on shared-antenna cases, where the TX–RX isolation challenges are the largest. It is confirmed that from the OFDM radar processing perspective, limited TX–RX isolation is primarily a concern in the detection of static targets, while moving targets are inherently more robust to transmitter self-interference (SI). Properly tailored analog/RF and digital SI cancellation solutions for OFDM radars are also described and implemented and shown through RF measurements to be key technical ingredients for practical deployments, particularly from static and slowly moving targets’ point of view.

Full-Duplex OFDM Radar With LTE and 5G NR Waveforms: Challenges, Solutions, and Measurements

This paper presents a description of recent research and development in HF passive bistatic radar (HFPBR) based on DRM digital AM broadcasting at Wuhan University, China First, preliminary evaluation of its detection performance with special focus on the hybrid sky-surface wave propagation mode is introduced Then, DRM broadcasting signal analysis as a radar waveform and associated signal processing techniques are described, consisting of ambiguity function analysis, reference signal extraction, multipath clutter rejection, and target localization Finally, the experimental system and experimental data analysis are provided Initial results from field experiments show that DRM-based HFPBR with hybrid sky-surface wave is a promising system for wide area moving target detection and ocean remote sensing

An Experimental Study of HF Passive Bistatic Radar Via Hybrid Sky-Surface Wave Mode

Recently, the widespread availability of consumer grade drones is responsible for the new concerns of air traffic control. This paper investigates the feasibility of drone detection by passive bistatic radar (PBR) system. Wuhan University has successfully developed a digitally multichannel PBR system, which is dedicated for the drone detection. Two typical trials with a cooperative drone have been designed to examine the system's capability of small drone detection. The agreement between experimental results and ground truth indicate the effectiveness of sensing and processing method, which verifies the practicability and prospects of drone detection by this digitally multichannel PBR system.

Digital television based passive bistatic radar system for drone detection

Digital television signals are attractive illuminations of opportunity for the passive radars in the field of low altitude and slow speed target detection. The digital television standard permits reconstruction of a reference signal using the received signal in surveillance channel, which enables a single-antenna digital television based passive radar (SDPR) processing. This paper investigates the practical feasibility of a multistatic SDPR (MSDPR) for the drone detection. First, the detection range of the SDPR is analyzed in terms of signal processing procedures involving multipath energy, extracted reference signal purity, and receiving antenna. Second, according to the characteristics of the SDPR, the reference signal extraction is analyzed. In addition, considering that the SDPR cannot locate and track the detected target, a novel MSDPR processing method is proposed. The core idea of this method is to use the optimal reference signal extracted from the receiving station with the least interference as the shared reference signal in MSDPR, which can greatly improve the system detection capability. Finally, the small drone detection experiments using the MSDPR are presented. The theoretical considerations are demonstrated using the experimental data.

Experimental Research of Multistatic Passive Radar With a Single Antenna for Drone Detection

In digital radio mondiale (DRM)-based high frequency (HF) passive bistatic radar, target echoes need to be detected against the direct-path wave and multipath echoes. A novel approach is proposed to reject the direct-path wave and multipath echoes. The signal is projected into a subspace orthogonal to the clutter subspace carrier by carrier, exploiting the fact that the direct-path wave and multipath echoes at the same carrier in orthogonal frequency division multiplexing (OFDM) waveform are totally correlated. Simulation and experimental results following the theory analysis verify the performance of the new approach.

Multipath clutter rejection for digital radio mondiale-based HF passive bistatic radar with OFDM waveform

China mobile multimedia broadcasting (CMMB) is a digital television standard that works in a single-frequency network (SFN) at the ultrahigh frequency band and adopts orthogonal frequency division multiplex modulation with a cyclic prefix (CP-OFDM). This paper investigates the practical feasibility of a CMMB-based passive radar (CPR) in the multipath propagation environment. First, several aspects involving system considerations induced by propagation conditions and radar signal characteristics are analyzed, including power budget, dynamic range requirement, clutter rejection metrics, and ambiguity function analysis. Then targeting the problems caused by multipath signals in the complex propagation environment and corresponding processing techniques are discussed, covering reference signal extraction and clutter rejection, as well as, tracking and localization. The highlight is clutter rejection with subcarrier-based spatial adaptive processing (SSAP), which is matched with the features of CP-OFDM. The applicability of SSAP under severe channel response spread mainly contributed by SFN is specially considered. Last, a description of the experiments and the CPR systems developed by Wuhan University is given, together with results from previous field trials for low-altitude aircraft and ship detection.

Xianrong Wan

Papers

An Experimental Study of HF Passive Bistatic Radar Via Hybrid Sky-Surface Wave Mode

Digital television based passive bistatic radar system for drone detection

Experimental Research of Multistatic Passive Radar With a Single Antenna for Drone Detection

Multipath clutter rejection for digital radio mondiale-based HF passive bistatic radar with OFDM waveform

Experimental Research for CMMB-Based Passive Radar Under a Multipath Environment